A well known differential amplifier comprises a matched pair of bipolar transistors, a matched pair of load impedances and a current source. Each load impedance is connected between a voltage supply and a collector of a respective one of the transistors, and emitters of both transistors are connected to the current source. Differential inputs are applied to bases of the transistors, and differential outputs are taken from the collectors of the transistors.
Two such differential amplifiers can be cross-coupled to construct a quadrature phase Voltage Controlled Oscillator (VCO) which oscillates with 90 degree phase shift across each differential amplifier and with 180 degree phase shift in the cross-coupled feedback loop.
Unfortunately, typical manufacturing process variations affect the oscillation frequency of such VCOs,so that such VCOs must be tuned after manufacture if oscillation at a particular oscillation frequency is required. W. P. Robins proposes tuning by means of varactor diodes which might be coupled between the collectors of the matched transistors in each differential amplifier (see Phase Noise in Signal Sources (Theory and Applications), Peter Peregrinus Ltd). Unfortunately, this method requires pretrimming of the load impedances to center the oscillation frequency because the tuning range achievable with varactor diodes alone cannot cope with circuit parameter variations which result from typical manufacturing process variations. Moreover, the resulting VCO typically has a fairly low quality factor (Q) due to nonlinearities in the circuit parameters.